1. Field of the Invention
This invention relates to a wet friction material that generates a torque by applying a high pressure to an opposite surface while being immersed in oil. In detail, this invention relates to a segment-type friction material made by joining friction material substrates that were cut into a segment piece onto one or both of surfaces of a core metal of a flat ring shape along an entire circumference thereof with an adhesive. Otherwise, this invention relates to a ring-type friction material made by pressing or cutting one or both of surfaces of a friction material of a ring shape that has ring-shaped friction material substrates joined onto one or both of surfaces of a core metal of a flat ring shape thereof with an adhesive so as to form a plurality of oil grooves in a radial direction.
2. Description of the Related Art
In recent years, a segment-type friction material has been developed as a wet friction material in order to improve a material yield with resultant cost reduction and to lessen a drag torque with resultant high fuel efficiency in a vehicle. The segment-type friction material is made by that friction material substrates that were cut into a segment piece along the flat ring shape are arranged and joined with an adhesive on a front surface of the core metal of the flat ring shape side-by-side along the entire circumference of the core metal with a gap to be an oil groove, and similarly another set of the segment-piece-shaped friction material substrates are joined with an adhesive to a rear surface of the core metal also. Such a segment-type friction material is applicable to an automatic transmission which may be referred to as “AT” hereafter of an automobile or the like or a friction material engaging device applied single or plural friction plates, which is used for a transmission of a motorcycle or the like.
As an example, a wet hydraulic clutch is used for an automatic transmission of an automobile or the like. The wet hydraulic clutch has a plurality of segment-type friction materials and a plurality of separator plates laid alternately on each other. Then, both the plates are contacted with a hydraulic pressure to transmit a torque. A lubricating oil (automatic transmission fluid, which may be referred to as “ATF” hereafter) is supplied to between both the plates for the purpose of absorbing a frictional heat generated when they are shifted from a disengaged state to an engaged state or preventing abrasion of friction material or the like. (“ATF” is a registered trademark of Idemitsu Kosan Co., Ltd.)
However, a distance between the segment-type friction material and a counterpart separator plate is set small so as to increase response of the hydraulic clutch. Moreover, a total area of an oil passage provided on the segment-type friction material is restricted so as to assure a sufficient torque transmission capacity when the hydraulic clutch is engaged. As a result, it becomes hard to discharge ATF remaining between the segment-type friction material and the separator plate when the hydraulic clutch is released and there was a problem that it generated a drag torque by ATF caused by a relative rotation of both the plates.
Therefore, in Japanese Laid Open Patent Publication No. 2001-295859, it is disclosed an invention regarding a wet friction member characterized in that division formation is carried out to oil grooves which are formed in between adjacent segment pieces and a distance of the oil groove makes narrow on the way from an inner peripheral side to an outer peripheral side. Hereby, ATF which runs from the inner peripheral side to the outer peripheral side is kept back at a point where the distance of the oil groove changes and certain ATF overflow and run to a front surface of the segment pieces, therefore it makes to be capable of reducing the drag torque as well as improving a heat resistance by improving a cooling effect of ATF.
Moreover, according to an invention described in Japanese Laid Open Patent Publication No. 2005-069411, two sides which are attached R in a reverse direction of the core metal of the segment piece are joined with an adhesive so as to be an inner periphery and an outer periphery. Therefore, it attaches R which goes up toward the direction of the oil groove to the inner periphery and a width of an outer peripheral opening portion of a gap to be an oil groove is larger than a width of an inner peripheral opening portion of the gap. Thus, it can significantly improve a discharging efficiency of ATF by idle running of the segment-type friction material and it can also substantially reduce the drag torque resulting from ATF.
Furthermore, according to an invention described in Japanese Laid Open Patent Publication No. 2005-282648, there is provided a segment-type friction material that when the segment-type friction material rotates in a disengaged state in case of being assembled into AT by cutting off a corner of the inner peripheral side of the segment piece at a predetermined angle, ATF is positively supplied to a friction surface of friction material substrates by that ATF which is supplied from the inner peripheral side touches a portion where the segment pieces were cut off, therefore it can restrain contact of a separator plate and the friction surface and considerably reduce the drag torque resulting from ATF.
Additionally, in Japanese Laid Open Patent Publication No. 2004-150449, in a segment-type friction material, it is disclosed an invention that it attaches R at four corners of the segment piece or it chamfers the four corners of the segment piece so as to prevent the segment piece from peeling by increasing a hydraulic pressure of ATF which touches a side surface of the segment piece (surface which consists of oil grooves). As a result, it is considered to be able to obtain a reduction effect of the drag torque resulting from ATF in case that shapes of the four corners of the segment piece are appropriate, while it significantly improves the peeling resistance.
However, in the technique described in the above patent publication No. 2001-295859, No. 2005-069411 and No. 2005-282648, it is assumed that ATF is supplied from the inner peripheral side of the core metal of the segment-type friction material. In an actual equipment, in case of a specification that there is no ATF supply from a hub hole or in case that a stirred torque increases by generating oil retained at the outer periphery, the drag torque reduction effect is not enough yet, and there is a problem that it is hard to bring a significant high efficiency in a vehicle.
Additionally, in the technique described in the above patent publication No. 2004-150449, it aims only to prevent the segment piece from peeling, therefore R or chamfering of four corners of the segment piece described in this embodiment is too small to obtain a drag torque reduction effect and it does not show an appropriate range of a size of R or chamfering of the four corners of the segment piece to reduce the drag torque. Moreover, there is a problem that all of the four corners of the segment piece are needed to make R processing or chamfering processing, which is not always required for the drag torque reduction.
As a result, an object of this invention is to provide a wet friction material that can certainly obtain a more significant drag torque reduction effect even in case that the stirred torque increases by generating oil retained at the outer periphery or in case of a specification that there is no lubricating oil supply from the inner peripheral side.